Field of the Invention
The present invention relates to a heat-curable epoxy resin composition for optical semiconductor element encapsulation; and an optical semiconductor device obtained by encapsulating an optical semiconductor element(s) using the same.
Background Art
Optical semiconductor elements such as LEDs (Light Emitting Diode) have been used in various indicators and light sources such as those for street displays, automobile lamps and residential lightings. Moreover, products using such optical semiconductor element are being rapidly developed in many fields such that carbon dioxide reduction and energy conservation are made available.
As an encapsulation material for encapsulating various optical semiconductor elements such as LEDs, there has long been used a heat-curable epoxy resin containing an epoxy resin such as a bisphenol A-type epoxy resin and/or an alicyclic epoxy resin; and an acid anhydride-based curing agent (JP-A-Hei 7-309927). This is because such heat-curable epoxy resin is low in cost, but is capable of forming a cured product superior in transparency, moisture resistance, heat resistance and light resistance.
In recent years, LEDs have become high-powered and highly bright, which requires an encapsulation material to be high-temperature resistant and light resistant. Therefore, it has become a trend to use a multifunctional epoxy resin and/or an alicyclic epoxy resin as an encapsulation material. However, a problem with these epoxy resins used as an encapsulation agent is that resin cracks will easily occur when using such epoxy resins to mold an optical semiconductor element(s). The reason for that is because these epoxy resins easily exhibit a decrease in strength due to their backbone structures (JP-A-Hei 9-213997 and JP-A 2000-196151).
As a solution to such problem, there has been reported an epoxy resin composition comprising a flexibility imparting agent such as polycarbonate polyol and polyester polyol. Although these epoxy resin compositions contribute to improving a reflow resistance, their physical properties tend to exhibit a decreased elastic modulus in a high-temperature state (Japanese Patent No. 5319567 and Japanese Patent No. 5675230).
Meanwhile, there exists a production method of a lamp for an automobile or the like where an optical semiconductor element on a lead frame is to be encapsulated and molded by an epoxy resin into the shape of a lens, followed processing such lead frame and then performing secondary molding through injection molding of a thermoplastic resin. There, since the thermoplastic resin will be heated to nearly 300° C. when performing secondary molding, an epoxy resin composition may be washed away by such thermoplastic resin if the epoxy resin composition exhibits low strength and elastic modulus in a high-temperature state. Further, when a glass-transition temperature is low, an epoxy resin cured product may directly reach a rubber state, which may cause the lens-shaped cured product to be washed away.